1. Field of Invention
The present invention relates to a novel method of venting a spray metal molding tool. More particularly the invention relates to a method of manufacturing a mold using a thermal spray process to produce a metal surface containing passages in the spray metal to vent or evacuate gas.
2. Description of Prior Art
The present invention is primarily intended for mold tools, such as a vacuum mold having vents or a multiplicity of holes in the forming surface to evacuate or supply gas in a mold. By way of example, vacuum forming of heat-softened sheets of plastic material is well known. Molds suitable for such vacuum forming are typically porous or have holes in the forming surface. A vacuum is applied behind the forming surface that evacuates air from between the mold surface and a heat-softened plastic sheet whereby bringing the sheet into conformance with the mold surface. Additionally, the forming surface may be grained or texturized to produce the desired surface on the plastic sheet.
The vacuum mold may incorporate cooling lines to cool the forming surface and the formed plastic sheet. The vent holes and cooling lines must be located to avoid interference. Care must be used in drilling the vent holes to avoid puncturing a cooling line. Depending on the construction of the mold, cooling lines can be incorporated in the molds metal surface or attached to the back of the metal surface or placed in the backing support structure. One method of constructing a vacuum mold utilizes a self supporting shell as the forming surface. The so-called shell is thin relative to the forming area. The shell can be made from metal, such as aluminum, nickel or kirksite with integral reinforcing ribs and cooling lines. Holes are then drilled through the mold and the mold is backed with a vacuum chamber.
There are several existing and excepted methods, described hereinafter, of creating a mold tool in part or whole with a venting surface and are comprised of, but not limited to:                (a) a mold produced from a cast shell or a machined metal block and drilled with a plurality of vents. Typically mold vents are holes, channels, valves, porous material inserts or other objects that permit gas to exit or enter the mold.        (b) a mold having a porous forming surface produced from a media filled epoxy as described in U.S. Pat. No. 4,952,355 to Seward et al. (1990);        (c) a mold surface having a porous sintered metal forming surface either machined or formed to the desired mold shape;        (d) a mold produced with a porous electroformed material as described in U.S. Pat. No. 5,632,878 to Kitano (1997);        (e) a mold produced from an electroformed or vapor deposited material and drilled with a plurality of vents.        
The existing methods of producing a mold surface with a plurality of vents are undesirable for several reasons including time, cost and surface detail capability. The disadvantages of the cast or the machined block mold in method (a) are the time and cost to drill a multiplicity of small vent holes in the surface. In order to place a small diameter vent hole in a cast or machined mold, a large clearance hole must be drilled from the back side of the mold and connected to the small vent hole drilled in the forming surface. This process is required for each vent hole and may take up to an hour for each vent. In a moderate sized mold, several hundred holes may be required to provide adequate venting. The porous epoxy mold in (b) lacks mold face durability and the ability to replicate small surface features and surface texture. The porous epoxy mold surface is generated by a filler media that must adhere to each other and provide interconnected porosity. The dilemma with the porous epoxy mold is that large media must be used to produce the porosity which reduces the strength of the mold. If smaller media is used, the mold can be stronger but lacks enough interconnected porosity. The porous sintered metal method in (c) is undesirable since the material is difficult to produce in bulk thicknesses, resulting in high material cost and long delivery times. The porous sintered material will further require machining or forming to the desired geometry. Machining and forming clogs or plugs the porosity, generating additional work to reactivate the pores. In many cases the pores cannot be unclogged completely which reduces porosity level thereby effecting venting capability. The method in (d) and (e) provides a venting metal surface to the desired shape, but the electroforming process can take several months to produce a mold surface. Although time is the most noted drawback of electroforming, the mandrel or model requirements add additional cost and time to the electroformed mold. An electroformed model must be conductive and made from a compatible material with the selected electroforming process.
Considering the shortcomings of the present technology, it would be desirable to create a new method of venting a mold at reduced cost and time.
A search of prior art found the following patents, relevant to the present invention:    U.S. Pat. No. 6,746,225 McHugh    U.S. Pat. No. 6,595,263 Grinberg et al.    U.S. Pat. No. 6,367,765 Wieder    U.S. Pat. No. 5,632,878 Kitano    U.S. Pat. No. 5,591,485 Weber et al.    U.S. Pat. No. 5,356,580 Clark et al.    U.S. Pat. No. 5,189,781 Weiss et al.    U.S. Pat. No. 4,952,355 Seward et al.    U.S. Pat. No. 4,165,062 Mitchell    U.S. Pat. No. 3,631,745 Walkey et al.    U.S. Pat. No. 3,077,647 Kugler    U.S. Pat. No. 2,629,907 Hugger